Abstract There is currently a lack of safety information regarding pharmaceutical use during pregnancy, posing a risk to both the mother and the developing fetus. This is primarily due to exclusion of pregnant women from clinical trials and the limitations of animal models to predict human tissue response. Therefore, there is a great demand for innovative methods to predict exposure-related developmental toxicity. Microphysiological systems (MPS) have emerged as in vitro tools to recapitulate human cellular and tissue response for drug discovery and pharmaceutical screening in a variety of organs, including the heart, lungs, and blood-brain barrier. Existing models of the placenta, which separates the maternal and fetal blood supply, fail to capture critical structural and functional hallmarks of the barrier. Therefore, we propose to build a placental MPS that mimics the native cellular microenvironment to better predict developmental exposure risk. We hypothesize that by recapitulating the placental microenvironment, we can develop in vitro barriers that are more structurally and functionally representative of the in vivo counterparts. First, we will optimize in vitro substrate compositions to potentiate placental trophoblast fusion by varying extracellular matrix and substrate elasticity across patho-physiological ranges found in the placenta. We will also quantify trophoblast fusion and hormone secretion to identify the optimal microenvironmental cues. Using these lessons learned, we will then build a placental barrier with a fused trophoblast and endothelial layer. We will evaluate our model by testing the relationship between trophoblast fusion and barrier permeability. Placental barrier function will be measured by quantifying the transport of fluorescent particles of varying size and surface chemistry. Finally, we will demonstrate the platform?s utility to predict clinically relevant pharmaceutical exposure by dosing the placental barrier with target drugs and chemicals, such as antidepressants. By combining this placental barrier with a previously developed cardiac MPS, we will probe the combined response of both placental barrier function and cardiac tissue organization and contractility. Upon completion of this project, we will gain a better understanding of the role of cellular microenvironment on trophoblast differentiation and fusion. Importantly, we hope to present a valuable and flexible tool that will enable researchers to simultaneously predict pharmaceutical and chemical exposure risk and developmental toxicity.